Why does Macbeth give the order to have Bunquo and his son, fleance, killed?

He was afraid someone would take his kingship. The witches said to watch out for his close friends. This lead him to become crazy and start doubting and killing his close companions. Also, many of Macbeth's friends were in favor of king Duncan and he might have been afraid that they would rebel against him and make Duncan's son king or that they would be unloyal to his leadership.